Answer 8—PCB trace width depends on how much voltage drop and temperature rise can be tolerated. Let’s say that the trace is 10 cm long and you want no more than 10-mV voltage drop.

The resistivity of solid copper is 1.7241 µW-cm. From this, you can calculate the required cross-sectional area of the trace: 

1 oz. per square foot corresponds to a copper thickness of 34 µm, or 0.0034 cm. Therefore, the trace width should be: 

Because 1 mil (0.001²) corresponds to 0.00254 cm, the trace should be at least 200 mils (0.2²) wide. Such a trace has about 1 mW per cm of length.

This also corresponds to a power dissipation of roughly 2 mW/cm². In order to calculate temperature rise, it is necessary to know what the effective thermal resistance to ambient (per unit area) is. This is affected by many factors such as the orientation of the board (convective effects), the presence of a fan or an enclosure, coatings on the copper (e.g., solder mask and silkscreen), etc.

Contributor: David Tweed